The invention relates to a method for producing a cast steel strip of preferably hot crack-sensitive or hot-brittle steel quality by means of a strip casting plant and to a strip casting plant for carrying out this method.
The strip casting plant is preferably a two-roll casting plant with two casting rolls arranged with their axes of rotation in a horizontal plane.
In contrast to carbon steels with a low C-content (C-content <0.20% by weight, in particular C<0.08%) and austenitic steel qualities, carbon steels with higher C-contents, electric steels and, in part, also ferritic and martensitic steel qualities tend to hot brittleness or hot crack sensitivity in temperature ranges above approximately 1150-1250° C. and upward. Developed casting methods use a strip casting plant, in particular a two-roll casting plant, in which the steel strip leaves the strip forming cross section between the two casting rolls, what is known as the kissing point, vertically downward and forms a largely freely suspended strip loop or in a suspended arc that strip is transferred, without appreciable strip support, into a horizontal transport direction at a relatively great distance from the casting rolls. In the case of hot crack-sensitive steel qualities, this leads to increased (intercrystalline) crack formation on the strip surface. Especially due to the dead weight of the sagging hot steel strip and on account of oscillatory movements or stochastic movements and strip tensile forces, this tendency is promoted.
To avoid such harmful influences on the still hot steel strip, methods have already been proposed which avoid a freely suspended loop of this kind. In a casting method improved in this way, using a vertical two-roll casting plant, steel melt is conducted in a melt space formed by two casting rolls and two side plates. The slab shells formed in the melt space on cooled surface areas of the casting rolls are brought together in the strip forming cross section between the casting rolls to form an at least partly solidified steel strip. In this case, the cast steel strip is guided along the surface area of one of the two casting rolls from a vertical casting direction into an essentially horizontal transport direction and is supplied essentially horizontally on a transport device to a strip winding device and is wound there into a coil. A method of this type is already known from JP-A 1-087045.
In this known casting method, the steel strip guided over an arc of a quarter circle along one of the casting rolls is treated in a shaping manner by a working roll capable of being thrown onto the casting rolls. Strip tensile forces occurring in this case induce mostly fluctuating tensile stresses in the steel strip which react into the strip forming cross section, which is conducive to hot crack formation.
Two-roll casting plants with strip guidance along a surface area of one of the two casting rolls are already known, further, from JP-A 2-247049, JP-A 2-295649, JP-A 2-290651 and JP-A 1-133651. In these strip casting plants, the casting roll guiding the cast strip is always assigned a pressing roll or a driver roll, which presses the strip onto the casting roll and, with a strip tension being applied, is moved in close contact with the casting roll. Here, too, the strip tension may possibly react into the strip forming cross section and is likewise conducive to hot crack formation.